1. Field of the Invention
The present invention relates to a method for measuring a thermal stability of a high-density information magnetic recording medium and a thermal stability measuring apparatus.
2. Description of the Prior Art
Conventionally, recording and reproduction are widely carried out by using a magnetic recording medium such as a magnetic disc. In order to increase an information amount recorded on a magnetic recording medium, it is desired to carry out recording with a high density, which in turn requires a magnetic recording medium having a low noise characteristic. In order to achieve a low noise, it is necessary to reduce the crystal particle size of the magnetic film of the magnetic recording medium. However, when the crystal particle size is decreased to a certain level, there arise a problem of thermal fluctuation which lowers the magnetization stability. That is, if a recording or reproduction is carried out onto/from a magnetic recording medium under a high temperature, the magnetization becomes unstable, disabling to record or reproduce correctly.
In general, as a index representing stability of crystal particles constituting a magnetic film, KuV/kT is used, wherein Ku represents a magnetic anisotropic energy; V represents a crystal particle volume; k, a Boltzmann's constant; and T, temperature. This is clearly described in "Thermal instability at 10 Gbit/in.sup.2 magnetic recording", IEEE Transaction on magnetics, No. 30, pp. 4230 to 4232, 1994. According to the Arrhenius' equation, when KuV/kT=25, magnetization of 1/e (wherein e is a natural exponential) is obtained for 100 seconds. If the KuV/kT exceed s this value, the magnetic film shows superparamgnetism. That is, KuV/kT=25 is a critical value. This is described in the article "Tandem deposition of small metal particle composites" in Journal of Applied Physics, No. 60, p.2548, 1986.
Thus, a KuV/kT value serves as an index representing stability. The k is a constant and the Ku and V values are determined by the magnetic film characteristics, and the like. The KuV/kT value is changed according to a temperature T. More specifically, as the temperature T increases, the KuV/kT values decreases, lowering magnetization stability. Although the KuV/kT=25 is a critical value of superparamagnetism, the KuV/kT should be a greater value for practical use as an actual magnetic recording medium. Accordingly, it is desired to determine a practical temperature range for respective recording media.
The practical temperature range can be determined through evaluation of thermal stability (thermal fluctuation). For example, there can be considered a method of placing a magnetic recording/reproduction apparatus such as a hard disc drive (HDD) in an electric furnace, temperature of which is gradually increased while detecting a reproduction output from a magnetic recording medium. However, if the temperature is increased, a magneto-resistance (MR) head changes its characteristics. For example, the MR ratio (ratio of an absolute value of resistance with respect to a resistance change due to magnetism) is decreased and an adhesive is peeled out. Thus, there arise problems inside the HDD and it is difficult to directly evaluate the thermal stability.
To cope with this, there have been suggested some methods for checking the affect to the thermal stability: a method of checking a coercive force dependency on the temperature; and a method using the magnetic force microscopy (MFn). These method s are both disclosed in the article "Thermal stability of perpendicularly recorded information" In Japan Applied Magnetics Association periodical, Vol. 21, Supplement No. S1 (5.sup.th perpendicular magnetic recording symposium), pp. 187-191.
The method to determine a coercive force dependency on the temperature to measure the thermal stability is realized by measuring the coercive force in a temperature range of absolute temperature of 100 to 500 K so as to determine the temperature change ratio of the coercive force.
In the method using the MFM, a sample (magnetic recording medium) on which a recording magnetic pattern has been formed is heated, for example, at the temperature of 200 C. for 3 hours, after which an observation with the MFM is carried out. An MFM output observed immediately after a magnetic recording is compared to an MFM output after the three hours of heating to determine an attenuation quantity.
However, in the method of determining the coercive force dependency on the temperature, no actual reproduction is carried out from a magnetic recording and it is impossible to determine a thermal affect to a reproduction signal. Moreover, the coercive force temperature change ratio obtained by this method includes an affect from the crystal orientation and accordingly, it is impossible to determine the thermal stability itself. Furthermore, the coercive force is determined by gradually increasing the magnetic field applied to the magnetic recording medium until magnetism disappears. Even if an information is recorded with a particular recording density, demagnetization is caused by a great magnetic field (such as 5 to 15 kOe).
On the other hand, the measuring method using the MFM has problems as follows.
The main problem is that in this measuring method, the MFM output is determined for a magnetic recording medium not in a heated state to a high temperature but in a state cooled to a certain level when the affect from the heat is mitigated. This is because it is impossible to carry out an MFM measurement while the magnetic recording medium is at a high temperature with unstable magnetization. The recording is changed from a stable first state at a room temperature to an unstable second state at a high temperature and then returned to a quasi-stable third state at a room temperature when the MFM measurement is carried to compare the first state to the third state. That is, the unstable second state having the greatest problem in thermal stability is not subjected to the MFM measurement. It is impossible to use the temperature o the second state heated to a high temperature as the absolute temperature T of the thermal stability index KuV/kT. Accordingly, with this measurement method, it is impossible to evaluate the thermal stability in the state (second state) of a great absolute temperature T and a small thermal stability index KuV/kT.
Because the MFM measurement is carried out not in the second state of unstable magnetization at a high temperature but in the third state of quasi-stable magnetization, there is little difference from the first state of stable magnetization prior to the heating. Even after the second state of unstable magnetization due to a high temperature, there is almost no difference in the MFM output between the third state at a room temperature and the first state. Accordingly, judging from the results of this measurement, it cannot be seen that heating causes thermal instability. That is, it is difficult to detect this instability by employing this measurement method.